AND  PHYSIOLOaY  OF  THE  SPONOIAD^. 
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of  about  15  or  20  degrees.  The  apex  of  the  spiculum  expands  into  a large,  more  or 
less  circular,  disc  or  shield,  having  in  the  fully  developed  state  an  extremely  sinuous 
or  foliated  margin;  the  plane  of  the  shield  or  disc  being  at  about  right  angles  to  the 
line  of  the  shaft,  and  having  the  under  side  thickly  studded  with  tubercles,  which  are 
separate  in  the  young  spicula,  and  more  or  less  confluent  in  the  fully  developed  ones 
(Plate  XXIV.  figs.  32,  33,  34,  35,  36,  37,  and  38). 
In  an  early  stage  of  its  development,  the  peltate  apex  of  the  spiculum  is  irregularly 
circular,  and  entirely  devoid  of  the  complex  and  beautiful  sinuous  foliations  that  render 
the  adult  spicula  such  elegant  objects  (Plate  XXIV.  fig.  32).  As  the  development  pro- 
ceeds, it  assumes  a trilobular  shape,  and  the  margins  are  slightly  indented  or  serrated 
(fig.  33). 
In  a further  advanced  condition,  the  sinuation  of  the  margin  becomes  deeper  and 
more  complex,  as  represented  in  figs.  34  and  35,  until  at  last  it  becomes,  in  the  fully 
developed  peltate  apex,  so  deeply  and  uTegularly  sinuated  as  nearly  to  obliterate  all 
traces  of  its  original  trilobular  character  (fig.  36). 
Much  as  theii’  beautiful  foliated  apices  differ  from  that  of  an  expando-ternate  spicu- 
lum, they  are  in  reahty  but  an  extreme  development  of  that  form.  If  we  examine  them 
mounted  in  Canada  balsam,  with  a linear  power  of  about  300,  we  frequently  find  at  the 
junction  of  the  shaft  with  the  inner  surface  of  the  disc,  the  remains  of  three  central 
tubular  cavities  radiating  from  the  distal  termination  of  the  tubular  cavity  of  the  shaft, 
and  extending  to  about  the  line  of  its  extreme  diameter,  or  rather  beyond,  as  shown  in 
fig.  32,  which  represents  a nearly  cu’cular  disc  or  shield ; and  in  the  three-lobed  forms  in 
which  they  are  apparent,  it  will  be  observed  that  the  axial  line  of  each  lobe  is  conform- 
able with  one  of  the  three  rudimentary  tubular  cavities  radiating  from  the  distal  termi- 
nation of  the  shaft.  There  is  good  reason  therefore  to  believe  that  these  lobes  are  due 
to  lateral  expansions  of  the  radii  of  originally  a ternate  form  of  spiculum.  In  other 
cases  we  find  sometimes  one,  and  at  other  times  two  of  the  radiating  central  cavities 
bifurcating  at  theh’  terminations,  as  in  Plate  XXIV.  fig.  38,  which  represents  the  under 
surface  of  a fragment  of  one  of  these  spicula;  and  in  this  case  the  tubular  cavities 
extend  considerably  beyond  the  cu'cumference  of  the  distal  end  of  the  shaft.  This 
bifurcation  of  the  radiating  tubular  cavities  is  in  perfect  accordance  with  the  correspond- 
ing structures  in  the  furcato-patento-ternate  spicula  of  Pachymatisma  lAsteri,  Bowee- 
BANK,  M8.,  as  represented  in  Plate  XXIII.  figs.  46  and  47.  The  number  of  lobes  into 
which  the  margin  of  the  great  apical  disc  or  shield  is  divided  in  the  adult  condition,  is 
exceedingly  various,  and,  as  a matter  of  course,  would  be  influenced,  to  a certain  extent, 
by  the  amount  of  bifurcation  of  the  radial  tubular  cavities  of  the  apex ; and  thus  it  is 
probably  that  we  find  so  great  a variation  in  the  form  and  number  of  the  marginal  lobes 
of  these  beautiful  and  interesting  spicula. 
There  is  some  difficulty  in  deciding  whether  this  singular  form  of  spiculum  should  be 
classed  with  the  connecting  ones  or  with  those  of  the  dermis.  It  is  found  in  a very 
beautiful  but  minute  siliceous  sponge,  forming  a thin  film  on  the  base  of  a specimen  of 
